LED array with photodetector

ABSTRACT

Embodiments of the present invention provide for an array, and corresponding method of forming an array, that includes a plurality of light emitting devices. The light emitting devices are disposed over a substrate, and a photodetector detects light emitted through the substrate from the light emitting devices. Further, a substantially constant brightness may be maintained in a plurality of light emitting devices disposed over the upper surface of a substrate in an array. Light emitted through the substrate from each of the light emitting devices is measured, and the voltage level applied to each of the light emitting devices is varied to maintain a substantially constant brightness level of light emitted from the light emitting devices.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/693,022, filed Oct. 23, 2003, which issue as U.S. Pat. No. 8,264,431on Sep. 11, 2012, which is incorporated by reference as if fully setforth.

FIELD OF INVENTION

The present invention relates to an array of LEDs (light emittingdiodes). More specifically, the present invention relates to such anarray having LEDs and a photodetector.

BACKGROUND

Arrays may use LEDs within individual pixel elements. Such arrays may beused in flat panel displays. One problem with such displays is that theindividual LEDs in the display may degrade at different rates due todifferent amount of use and other factors. When the difference inluminescence efficiency of LEDs within neighboring pixels becomesgreater than a certain threshold, such as 5%, the pixel non-uniformitybecomes observable, and the display is considered aged.

An aged display as described above may be no longer usable even thoughall pixels may still be brightly luminescent due to the observablenon-uniformities in brightness between pixels. Such differential agingof pixels limits the life of a such a display to only a few months,making the displays not commercially viable.

Accordingly, techniques for correction of brightness levels of pixels tocompensate for such differential aging in such displays have beendeveloped. For example, the displays may be operated in a constantbrightness mode instead of a constant current or voltage mode. Thebrightness level of each LED pixel may be monitored, and the currentlevel may be adjusted to maintain a constant and uniform brightness foreach LED pixel. It has been proposed to monitor the brightness levelthrough the use of photodetectors, which are typically made from silicontechnology.

Such photodetectors can be fabricated on a silicon substrate, with theLED layers formed by the photodetectors. This fabrication involvescostly processing steps. For example, the metallic layer contacting theLEDs to the photodetector backplane must be transparent so that lightcan reach the photodetectors. This may reduce the forward quantumefficiency of the photodetectors by as much as 50%. Further, a distinctphotodetector must be fabricated to support each LED, since the distancebetween the LED layers and the photodetectors will be submicron scale.Also, with such an approach, the photodetector circuitry will have toshare the precious area on the substrate with active matrix devicedrivers that also occupy space on the silicon backplane.

SUMMARY

Embodiments of the present invention provide for an array, andcorresponding method of forming an array, that includes a plurality oflight emitting devices. The light emitting devices are disposed over asubstrate, and a photodetector detects light emitted through thesubstrate from the light emitting devices.

Embodiments of the present invention provide for a method formaintaining a substantially constant brightness in a plurality of lightemitting devices disposed over the upper surface of a substrate in anarray. The method measures light emitted through the substrate from eachof the light emitting devices, and varying the voltage level applied toeach of the light emitting devices to maintain a substantially constantbrightness level of light emitted from the light emitting devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an LED mounted on a substrate.

FIG. 2 is a diagram of an array of LEDs.

FIG. 3 is a diagram of an embodiment of the present invention.

FIG. 4 is a diagram of an embodiment of the present invention.

FIG. 5 is a diagram illustrating an array in accordance with anembodiment of the present invention.

FIG. 6 is a diagram illustrating an array in accordance with anembodiment of the present invention.

FIG. 7 is a diagram illustrating an array in accordance with anembodiment of the present invention.

FIG. 8 is a diagram illustrating a compensation generator circuit inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention provide for an array, a method offorming an array, and a method for maintaining a substantially constantbrightness in a plurality of light emitting devices disposed over theupper surface of a substrate in an array.

FIG. 1 illustrates an LED 100 formed on a transparent substrate 110. Thesubstrate 110 may be a standard substrate or a flexible substrate. TheLED may be any type of LED, such as an organic light emitting diode(OLED), a polymer light emitting diode (PLED), or a quantum dot lightemitting diode (QDLED), for example. An OLED is shown in FIG. 1 as anexample. The LED may include ITO (indium tin oxide) layer 120, metallayer 150, and sandwiched there between are HTL (hole transport layer)130, an ETL (electron transport layer) 140. A voltage applied betweenthe ITO layer 120 and the metal layer 150 causes current to flow therebetween, and the LED generates electroluminescent light, which isemitted through the transparent substrate 110. A portion of the light iswaveguided in the substrate 110, and is emitted from edges of thesubstrate 110. The present invention measures the light emitted throughthe substrate, as further described below.

FIG. 2 illustrates an array 200 that may be used in accordance withembodiments of the present invention. The array 200 includes a pluralityof LEDs, such as representative LED 220, that are formed over asubstrate 210. The array may be used to form a display, where individualLEDs are selectively activated to form images. The arrays illustrateherein are passive matrix arrays, but the invention is equallyapplicable to active matrix arrays. The array 200 may include additionalelements, as further described below.

FIG. 3 illustrates an LED 300 formed over a transparent substrate 310 inaccordance with embodiments of the present invention. An OLED is shownas a type of LED that may be used. The LED includes HTL 330 and ETL 340sandwiched between ITO layer 320 and metal layer 350. This embodimentuses photodetector 360 mounted on a side surface of the substrate 310,to measure light emitted through the substrate. The quantity of lightmeasured is used to generate a compensation factor used to compensatefor aging of the LED. As the LED ages and hence emits a lower quantityof light, the compensation factor is used to increase the voltage (andthe current) applied to the LED to maintain a substantially constantbrightness, as further explained below.

FIG. 4 illustrates an LED 400 formed over a transparent substrate 410 inaccordance with embodiments of the present invention. An OLED is shownas one type of LED that may be used. The LED includes HTL 430 and ETL440 sandwiched between ITO layer 420 and metal layer 450. Thisembodiment uses photodetector 460 mounted on a lower surface of thesubstrate 410, to measure light emitted through the substrate. Thephotodetector 460 is not dispose directly below the LED where thesurface emitted EL light emerges, but is instead disposed along an outerperiphery edge of the substrate 410. Like the embodiment illustrated inFIG. 3, the photodetector 460 measures a quantity of light that iswaveguided through the substrate 410. As an alternative to theembodiment illustrated in FIG. 4, the photodetector could be placed onan upper surface of the substrate 410, along the outer periphery edge ofsubstrate 410.

FIG. 5 illustrates an array 500 in accordance with an embodiment of thepresent invention. The array 500 includes a substrate 510 having LEDs(not shown in this view) formed over the substrate. Any desired numberof LEDs may be used in the array, and the number used may be setdepending on desired resolution and cost, for example. The array alsoincludes photodetectors 520 formed along edge surfaces 530 of thesubstrate 510. The edge surfaces may be substantially perpendicular tothe upper surface of the substrate. The embodiment illustrated in FIG. 5includes twenty-eight photodetectors, with 7 along each of the four edgesurfaces 530. More or less photodetectors 520 may be used. The morephotodetectors 520 that are used, the quicker that the array maydetermine calibration factors for each LED in the array 500, as furtherexplained below.

FIG. 6 illustrates an embodiment of an array 600 in accordance with thepresent invention. In this embodiment, the photodetectors 610 are formedalong an outer periphery edge 620 of the array 600. The outer peripheryedge 620 will generally be a non-display area so as not to interferewith images that may be displayed by the LEDs. The embodiment shown inFIG. 6 illustrates twenty eight photodetectors 610, but again any numberof photodetectors 610 may be used.

FIG. 7 illustrates an array 700 in accordance with embodiments of thepresent invention. The array 700 includes a substrate 710 over which aplurality of LEDs are formed. As illustrated in previous figures, eachof the LEDs is formed between ITO and metal layers acting as electrodesof the LEDs. Data/column electrodes 720 form the ITO layers of each LED,and scanning/row electrodes 725 form the metal layer of each LED. TheHTL and ETL layers are disposed between and at the intersections of thedata/column electrodes 720 and the scanning/row electrodes 725.

Multiplexer 730 is used to selectively address the scanning/rowelectrodes 725. Column drivers 740 are used to selectively drive thedata/column electrodes 720. A voltage is applied across a particular LEDin the array 700 when a scanning/row electrode and an intersectingdata/column electrode are addressed at the same time, causing thecorresponding LED to emit light. By selectively addressing the LEDs,images may be displayed.

The present invention compensates for aging effects of individual LEDsin the array by monitoring the light waveguided from individual pixelsthrough the substrate 710 of the array and comparing the measured lightto a reference value. If the measured light drops below the referencevalue, the voltage applied to the particular LED is increased tomaintain the proper brightness level for the LED.

A photodetector or a plurality of photodetectors 750 are used to measurethe light from individual LEDs. As explained above in conjunction withFIGS. 3-6, the photodetector or plurality of photodetectors may beplaced in various positions on the substrate 710 where there may measurethe light waveguided in the substrate, such as on an outer periphery ofthe upper or lower surface of the substrate 710, or on an edge surfaceor edge surfaces of the substrate 710. The photodetectors 750 feed themeasured light to a feedback or compensation factor generator circuit760, which will be further explained below in connection with FIG. 8.

The compensation factor generator 760 compares measured light values ofindividual LEDs to a reference value. If the measured value is differentfrom the reference value, a compensation factor may be generated andstored in memory array 780, where it may be applied to individual LEDsby feedback to the column drivers 740.

When the array is initially calibrated, which may occur in a factory, anexternal camera may be used to measure brightness values for each LED.The current value needed to generate a reference brightness value, forexample 100 photons, is recorded, and this may be done for each LED inthe array. Additionally, for a particular photodetector, a measurementmay be made as to how many of the photons are waveguided through thesubstrate to the photodetector. For example, if 1 milliamp of current isneeded to generate 100 photons as measured by the camera, 2 photons maybe measured by a particular photodetector.

When a calibration occurs at a later time, 1 milliamp of current isagain applied to the particular LED as a reference, and thephotodetector measures a brightness of the LED. If less than 2 photonsis measured, the compensation factor can be used to increase the voltage(and current) applied to the LED, and then a further measurement may bemade by the photodetector. The voltage may then be readjusted, and thisprocess may continue until the proper brightness is measured at thephotodetector, as further described below.

FIG. 8 illustrates a compensation factor generator circuit 800 andphotodetector 830 that may be used in conjunction with the presentinvention. This is only one possible compensation factor generatorcircuit that may be used, and those of skill in the art would be able todevelop alternative circuit that would work with the present invention.

The photodetector 830 includes a photodiode 820 that detects lightemitted form the LED 810 through the substrate of the array. Themeasured light value may be fed to comparator 840, which may compare themeasured value to a reference value. If the measured value is differentfrom the reference value, the output of the comparator will change theoptical output of the LED to compensation for the difference by applyinga different voltage across the LED. This correction may be sufficient,but alternatively this compensation process may be repeated at achieve amore precise correction. The photodetector can read the light value ofthe LED again to readjust the output of the comparator and this processmay continue until the input to the comparator from the photodetectormatches the reference value. At this point, the compensation factor maybe sent to the memory for storage and use when the array is operating.

The compensation factor may be applied to the corresponding LED uponselection of the particular column and row electrodes, so that thevoltage applied across the electrodes is changed to compensate for theaging or other changes in performance of the LED, and to maintain adesired brightness of the LEDs.

When the array is used to generate the compensation factors, the arraymay activate a single LED, make a light measurement and calculate acompensation factor for the LED, and then repeat this process for eachLED in the array until all of the compensation factors are calculated.Using this method, if the array has 1024×800 LEDs, 1024×800 separatemeasurements would be made.

As explained above, the present invention may use one photodetector or aplurality of photodetectors to measure light emitted and waveguidedthrough the substrate. The more photodetectors that are used, the fasterthe compensation factors for all LEDs in the array may be calculated. Asan alternative to calculating compensation factors for one LED at a timeas explained above, if twenty-eight photodetectors are used, thentwenty-eight compensation factors for twenty-eight LEDs may becalculated simultaneously. In this way, the present invention wouldallow twenty-eight compensation factors to be calculated simultaneouslytaking approximately 10 microseconds for an LED array. For a standarddisplay having 1024×800 pixel LEDs, all of the compensation factorscould be calculated in approximately 0.3 seconds.

The present invention could be used in a display that calculates newcompensation factors every time the display is turned on, atpredetermined time intervals, or when selected by a user. A problem thatcould be encountered is that external light could interfere with thecompensation process, but this should only be a factor if the externallight is extremely bright. In such a case, the array could be covered orclosed during calibration.

Several embodiments of the invention are specifically illustrated and/ordescribed herein. However, it will be appreciated that modifications andvariations on the invention are covered by the above teachings andwithin the purview of the appended claims without departing from thespirit and intended scope of the invention.

1. An array, comprising: a plurality of light emitting devices disposedon an upper surface of a transparent substrate, wherein each of theplurality of light emitting devices is individually addressed to displayan image; and at least one photodetector non-removably flush mounted onan external face of a side surface of the transparent substrate fordetecting a fraction of waveguided light emitted from the plurality oflight emitting devices, wherein the side surface being substantiallyperpendicular to the upper surface, and wherein some fraction ofwaveguided light is observable.
 2. The array of claim 1, furthercomprising at least one additional photodetector formed on a lowersurface of the transparent substrate for detecting another fraction ofwaveguided light emitted from the plurality of light emitting devices.3. The array of claim 1, further comprising at least one additionalphotodetector formed on an outer periphery edge of the upper surface fordetecting another fraction of waveguided light emitted from theplurality of light emitting devices.
 4. The array of claim 1, furthercomprising a feedback circuit that measures a brightness level for eachof the plurality of light emitting devices, and varies a voltage appliedto individual ones of the light emitting devices to maintain abrightness level of each of the plurality of light emitting devices at asubstantially constant level.
 5. The array of claim 4, wherein thefeedback circuit includes a compensation factor generator for generatinga compensation factor for each of the plurality of light emittingdevices and a memory array for storing the compensation factor for eachof the plurality of light emitting devices.
 6. A display comprising thearray of claim
 1. 7. The array of claim 1, wherein the at least onelight emitting device is one of an organic light emitting device, apolymer light emitting device or a quantum dot light emitting device. 8.The array of claim 1, wherein each of the plurality of light emittingdevices is selectively activated to display the image.
 9. An array,comprising: a plurality of light emitting devices disposed on an uppersurface of a transparent substrate, wherein each of the plurality oflight emitting devices is individually addressed to display an image;and at least one photodetector non-removably flush mounted on anexternal face of a surface of the transparent substrate for detecting afraction of waveguided light emitted from the plurality of lightemitting devices, wherein some fraction of waveguided light isobservable.
 10. The array of claim 9, further comprising at least oneadditional photodetector formed on an external face of another surfaceof the transparent substrate for detecting another fraction ofwaveguided light emitted from the plurality of light emitting devices.11. The array of claim 10, wherein the surface is a side surface and theanother surface is a lower surface.
 12. The array of claim 10, whereinthe surface and the another surface is at least two of a lower surface,an upper surface or a side surface.
 13. The array of claim 9, whereinthe surface is a side surface.
 14. The array of claim 9, furthercomprising a feedback circuit that measures a brightness level for eachof the plurality of light emitting devices, and varies a voltage appliedto individual ones of the light emitting devices to maintain abrightness level of each of the plurality of light emitting devices at asubstantially constant level.
 15. The array of claim 14, wherein thefeedback circuit includes a compensation factor generator for generatinga compensation factor for each of the plurality of light emittingdevices and a memory array for storing the compensation factor for eachof the plurality of light emitting devices.
 16. A display comprising thearray of claim
 9. 17. The array of claim 9, wherein the at least onelight emitting device is one of an organic light emitting device, apolymer light emitting device or a quantum dot light emitting device.18. The array of claim 9, wherein each of the plurality of lightemitting devices is selectively activated to display the image.
 19. Amethod for forming an array, comprising: forming a plurality of lightemitting devices disposed on an upper surface of a transparentsubstrate, wherein each of the plurality of light emitting devices isindividually addressed to display an image; and non-removably mountingat least one photodetector on a surface of the transparent substrate fordetecting a fraction of waveguided light emitted through the transparentsubstrate, wherein a detector side of the at least one photodetectorfaces the plurality of light emitting devices, and wherein some fractionof waveguided light is observable.
 20. The method of claim 19, furthercomprising: forming at least one additional photodetector on anothersurface of the transparent substrate for detecting another fraction ofwaveguided light emitted from the plurality of light emitting devices,wherein the surface and the another surface is at least two of a lowersurface, an upper surface or a side surface.